Security devices can detect events such as temperature fluctuation, movement, impact, and shock. A passive infrared sensor (PIR) is one type of security device that detects motion as it measures infrared (IR) energy radiating from objects in its field of view in conjunction with a pyroelectric detector and Fresnel lens array as the objects move in and out of the multiple detection zones created by the sensing elements in the pyroelectric detector and the multiple Fresnel elements in the array molded into the Fresnel lens. IR energy enters the security device through the front of the device, known as the “sensor face.” The sensor face of traditional security devices includes a front cover, which is an opaque cylindrical plastic shield comprising the greater portion of the face of the device, where the shield has a small window or opening. The window overlays a cylindrical Fresnel lens, which is an IR transparent polymer material. The Fresnel lens is comprised of an array of Fresnel lens elements and a border surrounding the array. Normally, the Fresnel lens is molded flat during manufacturing, then bent to fit the cylindrical conformation of the security device when installed. The Fresnel lens is traditionally trapped in place between the edges of the window in the front cover on the front surface and a bug-guard or lens retainer on the interior side. This bending of the Fresnel lens creates considerable residual stresses in the lens, which, if not securely trapped between the front cover and the bug-guard, would lead to the edges lifting up from the sides and corners of the device as the lens tries to return to its original molded flat shape. In a PIR where the lens is to comprise the entire front face of the device, the edges or border of the lens cannot be trapped behind the front cover as it must reside on top of the front cover. It was found that lens retention via the use of hooks and pinned flanges would result in the lifting of the edges, causing gaps or holes in the device wherein dust, bugs, and water can seep in, potentially damaging the device or shortening its lifetime. In addition, the lifting of the edges of the lens can lead to distortions in the detection of IR energy, causing the device to not work properly. Thus, there is a need for a security device that avoids these problems in the prior art. The present device has a unique lens structure and a more robust lens system retention means that reduces the residual stresses on the lens.